A committee of four police chiefs issued a nine-page report which said that Newtown officers "navigated the inevitable chaos created within the first few minutes of such a call, managed to piece together what was occurring, but were unable to intervene before the shooter took his own life."

Adam Lanza killed 26 people, including 20 first-graders, after shooting his way into the school through a front glass window. He fired 154 rounds, almost all from a Bushmaster. 223, in less than five minutes before killing himself.

It took Newtown police five minutes and 57 seconds to make entry to the school in two spots — a rear window and a boiler room door on the north side of the school. Some experts have questioned why it took them so long to go into the school.

Manchester Police Chief Marc Montminy who chaired the committee, said officers didn't have all of the facts when they arrived. They didn't know how many shooters there were, if there were injuries or who the people were running around the outside of the building.

Newtown police detained parent Christopher Manfredonia, who had come to the school to help make gingerbread men when the shooting started, on the playground. Manfredonia was running around to the side of the building to try and reach his daughter's first-grade classroom, which was toward the rear of the school, after hearing the gunshots.

The 911 recordings released earlier this week reveal while officers were initially chasing Manfredonia that Lanza was still shooting inside the building.

"They had every reason to believe there was more than one shooter and only after they determined he wasn't the shooter they changed their plans,'' Montminy said.

The committee had access to 911 calls, dispatch tapes, video from cruisers and officer's statements. They also visited the school in September and were allowed to go inside to trace Lanza's path. The Connecticut Police Chiefs Association was asked to review the shooting by Newtown Police Chief Michael Kehoe.

In an interview, Montminy said it is unclear exactly what Newtown officers were doing in the more than five minutes they were outside of the building other than responding to Officer Michael McGowan's call for assistance in arresting Manfredonia.

The report also discusses the officer's initial response to the 911 call that "someone was shooting" inside the Sandy Hook school. McGowan was the first officer to arrive at the scene, in just under three minutes. The next three officers arrived about 13 seconds later, and within eight minutes of the 911 call there were nine Newtown officers on scene, records show.

They parked in the front of the school near the baseball field, more than 300 yards from the front entrance that Lanza had blasted through. Those officers said they could hear gunfire as the exited their cars and have said they originally thought someone was firing at them from outside the school.

The report says officers followed the department's policy by parking away from the front of the building and also by not trying to enter through the front door, which many state police officers did when they arrived a few minutes later.

Based on gunshots heard on 911 calls investigators believe that Lanza killed himself at 9:40:03 when the last gunshot is heard on three different 911 calls. The report noted the suicide was about one minute after police arrived.

"While we cannot prove the shooter killed himself due to the police arrival, the history of like incidents suggests this may be the case,'' the report concludes.

"It is our hope that this comprehensive review will put to rest any of the erroneous media coverage about the response of the Newtown first responders," Kehoe said in a statement on Thursday. "This report provides a thorough review of the evidence and provides clarity and context after review from accurate and fully vetted information."